In a burner of a gas turbine fuel is burnt to produce hot pressurized mainstream gases which are led to a turbine stage where they, while expanding and cooling, transfer momentum to turbine blades thereby imposing a rotational movement on a turbine rotor. Mechanical power of the turbine rotor can then be used to drive a generator for producing electrical power or to drive a machine. However, burning the fuel leads to a number of pollutants in the exhaust gas which can cause damage to the environment.
One method to reduce pollutants is to provide thorough mixing of fuel and air prior to combustion. Usually the premixing of fuel and air in a gas turbine engine takes place by injecting fuel into an air stream in a swirling zone of a combustor which is located upstream from the combustion zone. The swirling produces a mixing of fuel and air before the mixture enters the combustion zone. The design point of fuel injection systems for stationary gas turbine engines is usually close to full load conditions, where reasonably low NOx values are achieved.
However, for power output requirements different from the design point, the rate of formation of nitrous oxides may increase significantly. At relatively low power modes, such as at light-off for example or at other determined burner conditions, a relatively rich fuel/air ratio is desired for initiating combustion and maintaining stability of the combustion, which is achieved with a pilot fuel injection.
The present invention addresses premix fuel systems when operating the gas turbine engine at different loads.
EP 0 592 717 B1 describes a gas-operated premixing burner for the combustion chamber of, for example, a gas turbine in which, within a premixing space, the fuel injected by means of a plurality of nozzles is intensively mixed with the combustion air prior to ignition, the nozzles being arranged around a burner axis. Within the premixing space additional fuel nozzles are provided in the region of the burner axis, which fuel nozzles can be supplied via a separate fuel conduit, with the result that, in order to influence the fuel profile at the outlet from the premixing burner in a specific manner, the fuel concentration in the region of the burner axis is greater than the average fuel concentration in the outlet plane of the premixing burner. The separate fuel conduit is provided with a control valve which can be shut off.
EP 0 974 789 B1 describes a method of operating a gas turbine in which a liquid fuel is burned in a combustion chamber and the hot combustion gases produced in the process are directed through the gas turbine, and in which method the liquid fuel is fed to the combustion chamber via a plurality of controllable burners working in parallel and is sprayed into the combustion chamber via fuel nozzles, and the burners are divided into at least two groups of burners, and these groups are individually activated as a function of the operating state of the gas turbine.
EP 0 976 982 B1 describes a method of operating a gas turbine in which a gaseous fuel is sprayed via a plurality of burners, working in parallel and arranged on at least one concentric ring, into the combustion chamber and is burned there, and the hot combustion gases produced in the process are directed through the gas turbine, the burners are divided into at least two groups of burners, and these groups are activated individually as a function of the operating state of the gas turbine, the at least two groups, during the run-up of the gas turbine from the no-load idling operation to a full-load operation, being ignited and/or started up one after the other in at least two phases. At least one of the groups comprises the same burners as another group, the two groups differing only in the operating mode, of the burners, and the burners of the two groups being operated within a moderate load range in two operating modes.
GB 2 242 734 A describes a combustion assembly including a combustor having inner and outer liners, and pilot stage and main stage combustion means disposed between the liners. A turbine nozzle is joined to downstream ends of the combustor inner and outer liners and the main stage combustion means is close coupled to the turbine nozzle for obtaining short combustion residence time of main stage combustion gases for reducing NOx emissions. The combustion assembly includes first and second pluralities of circumferentially spaced fuel injectors for pilot stage and main stage combustion. Main injectors are for lean main injection only and pilot injectors are for rich pilot injection only.